[wp-trac] [WordPress Trac] #29351: Add a function for stampedes transient caching

[WordPress Trac]

#29351: Add a function for stampedes transient caching
----------------------------+-----------------------------
 Reporter:  hypertextranch  |      Owner:
     Type:  enhancement     |     Status:  new
 Priority:  normal          |  Milestone:  Awaiting Review
Component:  Cache API       |    Version:  trunk
 Severity:  normal          |   Keywords:
  Focuses:  performance     |
----------------------------+-----------------------------
 A common caching pattern that's used by WordPress and developers
 everywhere look something like this:

 {{{
 if ( false === ( $value = get_transient( $cache_key ) ) ) {
   // A
   // Bunch
   // Of
   // Code
   // $value = ...

   set_transient( $cache_key, $value, 300 );
 }
 }}}

 The downside of this approach is when the cache is cold or when the cache
 expires there's a stampede effect as multiple requests try and refresh the
 cache at the same time. Instead if we had a function that allowed for soft
 expirations we could do something like the following:

 {{{
 $some_slow_func = function() {
   // A
   // Bunch
   // Of
   // Code
   // $value = ...

   return $value;
 }

 $value = get_set_soft_transient(
   $cache_key,
   $some_slow_func,
   300,
   'Some fake initial stale value'
 );
 }}}

 This approach abstracts out the cache refresh logic into another function
 which can result in slightly cleaner code and can be used to prevent cache
 stampedes.

 Some other optimizations / changes that we could do with this approach
 that's not yet implemented in the patch:
 * Run all cache refresh functions async via wp_cron so even the unlucky
 winner that gets to trigger the cache refresh will get stale data (fast).
 * Better locking so that only one request will refresh the cache. (Right
 now a couple requests could happen to refresh at the same time on a very
 busy site.)
 * Perhaps prevent caching of a `false` value or one that equates to the
 default passed in?
 * Use filters instead of PHP callbacks to make this more WordPress like.
 * Also build out `get_set_soft_site_transient()`.

 This is inspired by the stampedes cache @gibrown wrote for some of
 WordPress.com and the @mboynes talk at WordCamp Boston.

 /cc @betzster

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Ticket URL: <https://core.trac.wordpress.org/ticket/29351>
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